Glyphosate is well known in the art as an effective post-emergent foliar-applied herbicide. In its acid form, glyphosate has a structure represented by formula (1):
and is relatively insoluble in water (1.16% by weight at 25° C.). For this reason it is typically formulated as a water-soluble salt.
Monobasic, dibasic and tribasic salts of glyphosate can be made. However, it is generally preferred to formulate glyphosate and apply glyphosate to plants in the form of a monobasic salt. The most widely used salt of glyphosate is the mono(isopropylammonium), often abbreviated to IPA, salt. Commercial herbicides of Monsanto Company having the IPA salt of glyphosate as active ingredient include Roundup®, Roundup® Ultra, Roundup® UltraMax, Roundup® Xtra and Rodeo® herbicides. All of these are aqueous solution concentrate (SL) formulations and are generally diluted in water by the user prior to application to plant foliage. Another glyphosate salt which has been commercially formulated as SL formulations include the mono(trimethylsulfonium), often abbreviated to TMS salt, used for example in Touchdown® herbicide of Syngenta. Various salts of glyphosate, methods for preparing salts of glyphosate, formulations of glyphosate or its salts and methods of use of glyphosate or its salts for killing and controlling weeds and other plants are disclosed in U.S. Pat. No. 4,507,250 to Bakel, U.S. Pat. No. 4,481,026 to Prisbylla, U.S. Pat. No. 4,405,531 to Franz, U.S. Pat. No. 4,315,765 to Large, U.S. Pat. No. 4,140,513 to Prill, U.S. Pat. No. 3,977,860 to Franz, U.S. Pat. No. 3,853,530 to Franz, and U.S. Pat. No. 3,799,758 to Franz. The aforementioned patents are incorporated herein in their entirety by reference.
Among the water soluble salts of glyphosate known in the literature, but not known to be used commercially, is the potassium salt, having a structure represented by formula (2):
in the ionic form predominantly present in aqueous solution at a pH of about 4. This salt is disclosed, for example, by Franz in U.S. Pat. No. 4,405,531 cited above, as one of the “alkali metal” salts of glyphosate useful as herbicides, with potassium being specifically disclosed as one of the alkali metals, along with lithium, sodium, cesium and rubidium. Example C discloses the preparation of the monopotassium salt by reacting the specified amounts of glyphosate acid and potassium carbonate in an aqueous medium.
Very few herbicides have been commercialized as their potassium salts. The Pesticide Manual, 11th Edition, 1997, lists as potassium salts the auxin type herbicides 2,4-DB ((2,4-dichlorophenoxy)butanoic acid), dicamba (3,6-dichloro-2-methoxybenzoic acid), dichlorprop (2-(2,4-dichlorophenoxy)propanoic acid), MCPA ((4chloro-2-methylphenoxy)acetic acid), and picloram (4amino-3,5,6-trichloro-2-pyridinecarboxylic acid), the active ingredient of certain herbicide products sold by DowElanco under the trademark Tordon.
The solubility of glyphosate potassium salt in water is recorded in pending application Ser. No. 09/444,766, filed Nov. 22, 1999, the entire disclosure of which is incorporated herein by reference. As disclosed therein, glyphosate potassium salt has a solubility in pure water at 20° C. of about 54% by weight, that is, about 44% glyphosate acid equivalent (a.e.) by weight. This is very similar to the solubility of the IPA salt. Concentrations expressed as percent by weight herein relate to parts by weight of salt or acid equivalent per 100 parts by weight of solution. Thus a simple aqueous solution concentrate of glyphosate potassium salt can readily be provided at a concentration of, for example, 44% a.e. by weight, comparable to that commercially obtainable with glyphosate IPA salt, as in the aqueous solution concentrate available from Monsanto Company under the name D-Pak. Somewhat higher concentrations can be obtained by slight over neutralization, 5 to 10% for example, of an aqueous solution of glyphosate potassium salt with potassium hydroxide.
A major advantage of the IPA salt over many other salts of glyphosate has been its compatibility in aqueous solution concentrate formulations with a wide range of surfactants. As used herein, the term “surfactant” is intended to include a wide range of adjuvants that can be added to herbicidal glyphosate compositions to enhance the herbicidal efficacy thereof, as compared to the activity of the glyphosate salt in the absence of such adjuvant, stability, formulability or other beneficial solution property, irrespective of whether such adjuvant meets a more traditional definition of “surfactant.”
Glyphosate salts generally require the presence of a suitable surfactant for best herbicidal performance. The surfactant can be provided in the concentrate formulation, or it can be added by the end user to the diluted spray composition. The choice of surfactant has a major bearing on herbicidal performance. For example, in an extensive study reported in Weed Science, 1977, volume 25, pages 275–287, Wyrill and Burnside found wide variation among surfactants in their ability to enhance the herbicidal efficacy of glyphosate, applied as the IPA salt.
Beyond some broad generalizations, the relative ability of different surfactants to enhance the herbicidal effectiveness of glyphosate is highly unpredictable.
Surfactants tending to give the most useful enhancement of glyphosate herbicidal effectiveness are generally but not exclusively cationic surfactants, including surfactants which form cations in aqueous solution or dispersion at pH levels of around 4–5 characteristic of SL formulations of monobasic salts of glyphosate. Examples are long-chain (typically C12 to C18) tertiary alkylamine surfactants and quaternary alkylammonium surfactants. An especially common tertiary alkylamine surfactant used in aqueous solution concentrate formulations of glyphosate IPA salt has been the very hydrophilic surfactant polyoxyethylene (15) tallowamine, i.e., tallowamine having in total about 15 moles of ethylene oxide in two polymerized ethylene oxide chains attached to the amine group as shown in formula (3):
wherein R is a mixture of predominantly C16 and C18 alkyl and alkenyl chains derived from tallow and the total of m+n is an average number of about 15.
For certain applications, it has been found desirable to use a somewhat less hydrophilic alkylamine surfactant, such as one having less than about 10 moles of ethylene oxide, as suggested in U.S. Pat. No. 5,668,085 to Forbes et al., for example polyoxyethylene (2) cocoamine. That patent discloses illustrative aqueous compositions comprising such a surfactant together with the IPA, ammonium or potassium salts of glyphosate. The highest concentration of glyphosate in the potassium salt formulations shown in Table 3 of the '085 patent is 300 g glyphosate a.e./l, with a weight ratio of glyphosate a.e. to surfactant of 2:1.
A class of alkoxylated alkylamines is disclosed in WO OO/59302 for use in herbicidal spray compositions. Potassium glyphosate solutions including various Jeffamine™ EO/PO propylamines or propyldiamines are described therein.
A wide variety of quaternary ammonium surfactants have been disclosed as components of aqueous solution concentrate formulations of glyphosate IPA salt. Illustrative examples are N-methylpolyoxyethylene (2) cocoammonium chloride, disclosed in European Patent No. 0274369, N-methylpolyoxyethylene (15) cocoammonium chloride, disclosed in U.S. Pat. No. 5,317,003, and various quaternary ammonium compounds having formula (4):(R1)(R2)(R3)N+—CH2CH2O—(CH2CH(CH3)O)nHCl−  (4)where R1, R2and R3 are each C1-3 alkyl groups and n is an average number from 2 to 20, disclosed in U.S. Pat. No. 5,464,807.
PCT Publication No. WO 97/16969 discloses aqueous solution concentrate compositions of glyphosate, in the form of the IPA, methylammonium and diammonium salts, comprising a quaternary ammonium surfactant and an acid salt of a primary, secondary or tertiary alkylamine compound.
Other cationic surfactants which have been indicated as useful in aqueous solution concentrate compositions of glyphosate salts include those disclosed in PCT Publication No. WO 95/33379. It is further disclosed in PCT Publication No. WO 97/32476 that highly concentrated aqueous compositions of glyphosate salts can be made with certain of these same cationic surfactants, with the further addition of a defined component that enhances stability of the compositions. Glyphosate salts exemplified therein are the IPA salt and the mono- and diammonium salts.
A class of alkyletheramine, alkyletherammonium salt and alkyletheramine oxide surfactants has been disclosed in U.S. Pat. No. 5,750,468 to be suitable for preparation of aqueous solution concentrate formulations of various glyphosate salts, the potassium salt being included in the list of salts mentioned. It is disclosed therein that an advantage of the subject surfactants when used in an aqueous composition with glyphosate salts is that these surfactants permit the glyphosate concentration of the composition to be increased to very high levels.
Anionic surfactants, except in combination with cationic surfactants as disclosed in U.S. Pat. Nos. 5,389,598 and 5,703,015, are generally of little interest in SL formulations of glyphosate IPA salt. The '015 patent discloses a surfactant blend of a dialkoxylated alkylamine and an anionic eye irritancy reducing compound. The surfactant blend is disclosed as being suitable for preparation of aqueous solution concentrate formulations of various glyphosate salts, the potassium salt being included in the list of salts mentioned. Concentrates of the '015 patent contain from about 5 to about 50%, preferably about 35% to about 45% glyphosate a.i. and from about 5 to about 25% surfactant. Further, PCT Publication No. WO 00/08927 discloses the use of certain polyalkoxylated phosphate esters in combination with certain polyalkoxylated amidoamines in glyphosate containing formulations. Potassium is identified as one of several salts of glyphosate noted as being “suitable.”
Nonionic surfactants are generally reported to be less compatible with glyphosate than cationic or amphoteric surfactants when used as the sole surfactant component of SL formulations of glyphosate; exceptions appear to include certain alkyl polyglucosides, as disclosed for example in Australian Patent No. 627503. Other nonionics that have been disclosed as useful with glyphosate include polyoxyethylene (10–100) C16-22 alkylethers, as disclosed in PCT Publication No. WO 98/17109. Other nonionic surfactants are generally mixed with cationic surfactants to form a compatible surfactant system for use in liquid herbicidal concentrates. However, cationic/nonionic surfactant systems generally do not provide acceptable low temperature storage stability. Concentrates containing these surfactant systems can crystallize at temperatures at or below about 0° C., limiting the use of such concentrates in cold climates.
Glyphosate concentrates containing nonionic alkylether and cationic amine surfactants are described in U.S. Pat. No. 6,245,713. The surfactant mixture is said to enhance biological effectiveness of the glyphosate and provide enhanced rainfastness. Suitable glyphosates for use in the concentrates include sodium, potassium, ammonium, dimethylammonium, IPA, monoethanolammonium and TMS glyphosate salts. This patent is incorporated herein in its entirety by reference.
It is likely that serious consideration of glyphosate potassium salt as a herbicidal active ingredient has been inhibited by the relative difficulty in formulating this salt as a highly concentrated SL product together with preferred surfactant types. For example, a widely used surfactant in glyphosate IPA salt compositions, namely polyoxyethylene (15) tallowamine of formula (3) above, is highly incompatible in aqueous solution with glyphosate potassium salt. Further, PCT Publication No. WO 00/15037 notes the low compatibility of alkoxylated alkylamine surfactants in general with high-strength glyphosate concentrates. As disclosed therein, in order to “build in” an effective level of surfactant, an alkylglycoside surfactant is used in combination with an alkoxylated alkylamine surfactant to obtain high-strength concentrates containing the potassium salt of glyphosate.
The addition of such alkylglycosides resulted in higher viscosity formulations (as compared to formulations without alkylglycosides). Such an increase in the viscosity of these high-strength formulations is undesirable for various reasons. In addition to being more difficult to conveniently pour from the container or to wash residues therefrom, the deleterious effects resulting from higher viscosity formulations is more dramatically observed with respect to pumping requirements. Increasing volumes of liquid aqueous glyphosate products are being purchased by end-users in large refillable containers sometimes known as shuttles, which typically have an integral pump or connector for an external pump to permit transfer of liquid. Liquid aqueous glyphosate products are also shipped in bulk, in large tanks having a capacity of up to about 100,000 liters. The liquid is commonly transferred by pumping to a storage tank at a facility operated by a wholesaler, retailer or cooperative, from which it can be further transferred to shuttles or smaller containers for onward distribution. Because large quantities of glyphosate formulations are purchased and transported in early spring, the low temperature pumping characteristics of such formulations are extremely important.
When such alkylglycosides (e.g., Agrimul™ APG-2067 and 2-ethyl-hexyl glucoside) are added to a glyphosate concentrate, the concentrate is dark brown in color. It is desirable for a glyphosate concentrate to be lighter in color than the alkylglycoside-containing concentrates as disclosed in WO 00/15037, which have a color value of about 10 to 18 as measured by a Gardner colorimeter. When dye is added to a glyphosate concentrate having a Gardner color of 18, the concentrate remains dark brown in color. Concentrates having a Gardner color value of 10 are difficult to dye a wide variety of colors, for example blue, green, red or yellow, as is often desired to distinguish the glyphosate product from other herbicidal products.
It would be desirable to provide a storage-stable aqueous concentrate composition of the potassium salt of glyphosate having an agronomically useful surfactant content, or that is “fully loaded” with surfactant. These formulations exhibit a reduced viscosity such that they may be pumped with standard bulk pumping equipment at 0° C. at rates of at least 7.5 gallons per minute, usually more than 10 gallons per minute and preferably greater than 12.5 gallons per minute. An “agronomically useful surfactant content” means containing one or more surfactants of such a type or types and in such an amount that a benefit is realized by the user of the composition in terms of herbicidal effectiveness by comparison with an otherwise similar composition containing no surfactant. By “fully loaded” is meant having a sufficient concentration of a suitable surfactant to provide, upon conventional dilution in water and application to foliage, herbicidal effectiveness on one or more important weed species without the need for further surfactant to be added to the diluted composition.
By “storage-stable,” in the context of an aqueous concentrate composition of glyphosate salt further containing a surfactant, is meant not exhibiting phase separation on exposure to temperatures up to about 50° C., and preferably not forming crystals of glyphosate or salt thereof on exposure to a temperature of about 0° C. for a period of up to about 7 days (i.e., the composition must have a crystallization point of 0° C. or lower). For aqueous solution concentrates, high temperature storage stability is often indicated by a cloud point of about 50° C. or more. Cloud point of a composition is normally determined by heating the composition until the solution becomes cloudy, and then allowing the composition to cool, with agitation, while its temperature is continuously monitored. A temperature reading taken when the solution clears is a measure of cloud point. A cloud point of 50° C. or more is normally considered acceptable for most commercial purposes for a glyphosate SL formulation. Ideally the cloud point should be 60° C. or more, and the composition should withstand temperatures as low as about −10° C., preferably as low as about −20° C., for up to about 7 days without crystal growth, even in the presence of seed crystals of the glyphosate salt.
A surfactant that is described herein as “compatible” with a glyphosate salt at specified surfactant and glyphosate a.e. concentrations is one that provides a storage-stable aqueous concentrate as defined immediately above containing that surfactant and salt at the specified concentrations.
Users of liquid herbicidal products typically meter the dosage by volume rather than by weight, and such products are usually labeled with directions for suitable use rates expressed in volume per unit area, e.g., liters per hectare (l/ha) or fluid ounces per acre (oz/acre). Thus the concentration of herbicidal active ingredient that matters to the user is not percent by weight, but weight per unit volume, e.g., grams per liter (g/l) or pounds per gallon (lb/gal). In the case of glyphosate salts, concentration is often expressed as grams of acid equivalent per liter (g a.e./l).
Historically, surfactant-containing glyphosate IPA salt products such as Roundup® and Roundup® Ultra herbicides of Monsanto Company have most commonly been formulated at a glyphosate concentration of about 360 g a.e./l. The surfactant-containing glyphosate TMS salt product Touchdown® of Zeneca has been formulated at a glyphosate concentration of about 330 g a.e./l. Products at lower a.e. concentration, i.e., more dilute, are also sold in some markets, but carry a cost penalty per unit of glyphosate they contain, primarily reflecting packaging, shipping and warehousing costs.
Further benefits in cost savings and in convenience to the user are possible if a “fully loaded” aqueous concentrate composition, or at least one having an agronomically useful surfactant content, can be provided at a glyphosate concentration of at least about 320 g a.e./l, 340 g a.e./l, or significantly more than 360 g a.e./l, for example at least about 420 g a.e./l or more, or at least 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, 660 g a.e./l or more.
At very high glyphosate a.e. concentrations such as these, a significant problem normally occurs. This is the difficulty in pouring and/or pumping of the aqueous concentrate arising from the high viscosity of the concentrate, especially as manifested at low temperatures. It would therefore be highly desirable to have a highly concentrated aqueous solution of glyphosate potassium salt fully loaded with an agronomically useful surfactant, such formulation preferably being less viscous than glyphosate potassium salt formulations containing alkylglycoside surfactants, such as those disclosed in PCT Publication No. WO 00/15037.
As will be clear from the disclosure that follows, these and other benefits are provided by the present invention.